In many cases, it is desirable for a doctor to be able to monitor a patient's blood pressure over long periods of time. Numerous blood pressure monitoring systems have been developed to provide automatic measurement of the patient's blood pressure at predetermined time intervals. In general, these monitoring systems comprise a transducer which measures pressure variations in a pressurized cuff attached to the patient's arm or flnger. The output from the transducer is processed by a computer algorithm to obtain an indication of the patient's blood pressure.
The blood pressure measured by the transducer using a conventional monitoring system may fluctuate signiflcantly due to a number of physiological and environmental factors. For example, respiration and physical movement of the patient may create motion artifacts and other spurious signals which must be filtered from the transducer output in order to obtain an accurate representation of the patient's blood pressure. Most monitoring systems incorporate sophisticated algorithms to remove these undesired signals and thus ensure the integrity of the blood pressure reading.
In addition to undesired signals such as those discussed above, an additional error can be introduced by pressure differentials which are related to the position of the limb to which the transducer is attached. This error is related to a hydrostatic pressure differential between the blood in the heart and the column of blood in the blood vessel where the pressure measurement is being made.
The importance of correcting for this hydrostatic pressure error depends on the reason for taking the patient's blood pressure and on the situation in which the measurement is being made. As a general principle, the preferred blood pressure measurement is the central aortic systolic and diastolic pressure. However, it is not possible to measure central aortic pressure by indirect measurement techniques, such as those utilizing a transducer attached to the patient's limb. The blood pressure measured in the patient's limb is different from the central aortic pressure because of pulse pressure differentials created by the peripheral arteries. Such pressure differentials are especially prevalent in blood pressure measurements made in the lower arm and the legs.
The pressure differentials related to indirect blood pressure measurement are well understood and the error caused by these differentials are simply factored into the blood pressure measurement. The errors related to hydrostatic pressure differentials, however, can cause significant difflculties in certain diagnostic procedures. In particular, such errors are important in situations where changes in the mean blood pressure are important to obtain a correct diagnosis.
In some situations, such as monitoring a patient in the operating room, it is important to monitor only significant changes in the patient's blood pressure. In other cases, however, it is important that the actual blood pressure be monitored. For example, in the diagnosis of hypertension, the patient's actual blood pressure is monitored over a long period of time. The data obtained during this measurement period are compared to the blood pressures contained in a data base representative of a large population. The actual blood pressure measurements used in this diagnosis are normally taken while the patient is relaxed and sitting or lying on his back with the cuff at heart level. If the position of the pressure transducer relative to the heart is changed during the series of measurements, a hydrostatic pressure of more than 10 mm of mercury can be created. Such a pressure differential is significant enough to cause an error in the diagnosis.
Prior automatic blood pressure monitoring systems have not incorporated a means for compensating for the error caused by the position of the transducer. It is important, therefore, that an effective blood pressure monitoring system be provided which can compensate for the hydrostatic pressure differential related to changes in the position of the transducer relative to the position of the patient's heart.